The present invention relates to a method for cleaning surface of a steel plate during cold rolling and, more particularly, to a cleaning method for removing from the surface of the steel plate the rolling oil and the metal powders generated inside the cold-rolling machine during the cold-rolling procedure so as to facilitate the subsequent processes.
Cold-rolled steel plates are often used for automobile bodies, shells of electrical equipment for homes and so on. After the cold-rolling process, processes of annealing, phosphate chemical conversion treatment and so on are performed. Since the cold rolling is performed under the presence of the rolling oil in the cold-rolling method of steel plates, the rolling oil and the metal powders from abrasion remain attached to the surfaces of the steel plates after the cold rolling. If annealing is performed while the rolling oil and the metal powders are attached to the surfaces of the steel plates, carbon and metal powders remain on the surfaces of the steel plates after the annealing. Thus, even if treatments such as the chemical conversion treatment are performed on such surfaces, the treated surfaces are not free from quality degradation, so that desirable products may not be obtained. Accordingly, it is general practice to perform immersion cleaning or electrolytic cleaning of the steel plates so as to obtain steel plates with clean annealed surfaces.
Electrolytic cleaning is generally performed at a solution temperature of over 80.degree. C. (over 60.degree. C. at least) and adopts an alkali water solution for providing electrical conductivity in the solution. The metal plate must be rinsed with water after the cleaning for removing the alkalis attached to the plate. Consequently, the equipment for performing these treatments becomes big in size and the processing cost of the steel plates greatly increases. This method is also defective in that a great amount of exhaust water which contains oil and alkali is generated from the equipment. The installation of the processing equipment is cumbersome and expensive.
The mill-clean method has also been proposed which attempts to decrease the amount of carbon precipitated on the surfaces of the steel plates during the annealing process by improving the composition of the rolling oil. However, in this method, part of the rolling oil which has entered the annealing atmosphere evaporates during the temperature rise of the annealing procedure to be dispersed outside the system with the annealing gas, and most of the rolling oil, in the presence of hydrogen, precipitates carbon on the surfaces of the steel plates by precipitation decomposed carbon of under the presence of hydrogen or by decomposing hydrocarbons to lower class hydrocarbons. The iron powder generated from the rolling remains adhered to the surfaces of the steel plates, adversely affecting the corrosion resistance of the painted products.
One object of the present invention is, therefore, to provide a cleaning method for steel plates according to which the equipment and procedure of the method are simple and the degree of cleaning obtainable is high.
Another object of the present invention is to provide a cleaning method for steel plates according to which a higher degree of cleaning can be performed at less cost.
A still further object of the present invention is to provide a cleaning method for steel plates which does not generate harmful exhaust water.
To the above and other ends, the present invention provides a cleaning method for steel plates according to which highly pressurized solution is sprayed on both surfaces of a strip steel plate immediately before and/or after the final rolling during the cold-rolling process of the strip steel plate, this highly pressurized solution sprayed under the conditions of: EQU (T).sup.3 .times.P&gt;3.5.times.10.sup.5
(where T is the temperature of the cleaning solution in .degree.C., and P is the pressure of the cleaning solution in kg/cm.sup.2) such that the higher the temperature and the pressure of the sprayed water, the more efficient the removal of the rolling oil and the metal powders from abrasion and the smaller the amount of the sprayed water required for this purpose.
By mixing abrasive grains in the spraying solution for cleaning the surfaces of the steel plates by abrasion, the cleaning may be performed with higher efficiency, the rolling oil and the metal powders from abrasion attached during the cold-rolling process are effectively removed, and the problems arising during the subsequent processes of the annealing are eliminated.
Further, by adding a rust inhibitor and/or a surface active agent to the highly pressurized solution, the surface may be made rustproof as the surface processing is performed, thereby facilitating the processes to follow.